1. Field of the Invention
The present invention relates to a process for synthesizing silicon nitride, and more particularly to a process for obtaining a silicon nitride powder in a high yield by the gas-phase process.
2. Description of the Prior Art
As a means for obtaining a silicon nitride powder by the gas-phase process, there has been generally employed a process which comprises reacting a silicon halide, such as silicon tetrachloride etc., with ammonia at a high temperature.
Since this process is an endothermic reaction, it has heretofore been required to impart the necessary heat of reaction to a reaction zone, such as a reactor etc., by external heating.
However, when such a process is employed, a silicon nitride synthesized on the inner wall of the reactor sticks thereto as an extremely hard deposit, which is not easily removed by scraping, and therefore, it not only causes a reduction in yield but also results in the deterioration of the reactor. Further, depending on the material used as the reactor, there are such disadvantages that on scraping the stuck silicon nitride, the material of the reaction zone is scraped away and incorporated as undesired impurities in the reaction product, and so forth.
Under such circumstances, we have been intensively studying on a means for reducing as much silicon nitride stuck within the reaction zone as possible for the purpose of improving the yield of silicon nitride obtained, and, as a result, have discovered that the sticking phenomenon of silicon nitride takes place in the stage where the reaction product from the starting materials is amorphous. It has now also been discovered that when producing silicon nitride by reacting a silicon halide with ammonia at a high temperature, such a sticking phenomenon can be effectively reduced by utilizing the heat generated by burning certain elements in the reactor to supply the heat of reaction.